Indium is supplied in the form of alloy targets with zinc and tin and paste as a material for transparent conductive films such as a semiconductor device and a touch panel and is widely used.
Ores incidentally contain indium in a very slight amount and also indium is unevenly distributed. So it is not easy to ensure a stable supply of indium. In addition, ores containing indium contain lead and zinc in a grossly excessive amount as compared to the amount of indium in many cases. Thus, when recovering indium there is a great problem with efficiently separating indium from lead and zinc.
Therefore, indium is obtained from ores, and furthermore indium is also obtained by recovering electronic circuit boards discarded in the city, defective items produced in production processes and other target materials which have been effectively finished and the like and recycling these. When the discarded electronic circuit boards and the like are recycled to obtain indium, however, there is still a problem with efficiently separating indium from lead and zinc.
As a method for separating indium from lead and zinc, a solution obtained by a two-stage neutralization treatment during the zinc leaching residue treatment step of zinc smelting is used as a solution to be extraction containing gallium and indium, and this solution to be extraction is adjusted to pH 2.4 to 3.6. Separately, a chelating agent is added to an organic solvent, and an extraction organic solvent having an organic phase in which a part of the protons in this chelating agent is substituted by an alkaline earth metal is also prepared. It is proposed that both these solutions be mixed to extract and isolate gallium and indium in the above solution to be extracted (see Patent Document 1). The method described in Patent Document 1, however, cannot be directly applied to a solution leached using an acid, for example, a low pH solution with as pH less than 2, and costs for as neutralizer to adjust pH, neutralization equipment, and much time for treatment are required.
It is also suggested that indium, gallium and zinc are each isolated using a chelating extraction agent having phosphinic acid as a ligand (see Patent Document 2). The invention described in Patent Document 2, however, has high selectivity to zinc in coexistence with cadmium. Accordingly, when it is required to separate indium from zinc out of members not using cadmium like modern electronic members, it cannot be said that indium is efficiently obtained.
Patent Document 1: Japanese Unexamined Patent Application, Publication No. 2000-212658
Patent Document 2: Japanese Unexamined Patent Application, Publication No. 2009-256291